@@ -138,11 +138,10 @@ associated with the source address of the indirect branch. Specifically,
the BHB might be shared across privilege levels even in the presence of
Enhanced IBRS.
-Currently the only known real-world BHB attack vector is via
-unprivileged eBPF. Therefore, it's highly recommended to not enable
-unprivileged eBPF, especially when eIBRS is used (without retpolines).
-For a full mitigation against BHB attacks, it's recommended to use
-retpolines (or eIBRS combined with retpolines).
+Previously the only known real-world BHB attack vector was via unprivileged
+eBPF. Further research has found attacks that don't require unprivileged eBPF.
+For a full mitigation against BHB attacks it is recommended to set BHI_DIS_S or
+use the BHB clearing sequence.
Attack scenarios
----------------
@@ -430,6 +429,21 @@ The possible values in this file are:
'PBRSB-eIBRS: Not affected' CPU is not affected by PBRSB
=========================== =======================================================
+ - Branch History Injection (BHI) protection status:
+
+.. list-table::
+
+ * - BHI: Not affected
+ - System is not affected
+ * - BHI: Retpoline
+ - System is protected by retpoline
+ * - BHI: BHI_DIS_S
+ - System is protected by BHI_DIS_S
+ * - BHI: SW loop
+ - System is protected by software clearing sequence
+ * - BHI: Syscall hardening
+ - Syscalls are hardened against BHI
+
Full mitigation might require a microcode update from the CPU
vendor. When the necessary microcode is not available, the kernel will
report vulnerability.
@@ -484,7 +498,11 @@ Spectre variant 2
Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
boot, by setting the IBRS bit, and they're automatically protected against
- Spectre v2 variant attacks.
+ some Spectre v2 variant attacks. The BHB can still influence the choice of
+ indirect branch predictor entry, and although branch predictor entries are
+ isolated between modes when eIBRS is enabled, the BHB itself is not isolated
+ between modes. Systems which support BHI_DIS_S will set it to protect against
+ BHI attacks.
On Intel's enhanced IBRS systems, this includes cross-thread branch target
injections on SMT systems (STIBP). In other words, Intel eIBRS enables
@@ -638,6 +656,21 @@ kernel command line.
spectre_v2=off. Spectre variant 1 mitigations
cannot be disabled.
+ spectre_bhi=
+
+ [X86] Control mitigation of Branch History Injection
+ (BHI) vulnerability. Syscalls are hardened against BHI
+ regardless of this setting. This setting affects the deployment
+ of the HW BHI control and the SW BHB clearing sequence.
+
+ on
+ unconditionally enable.
+ off
+ unconditionally disable.
+ auto
+ enable if hardware mitigation
+ control(BHI_DIS_S) is available.
+
For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
Mitigation selection guide
@@ -5838,6 +5838,17 @@
sonypi.*= [HW] Sony Programmable I/O Control Device driver
See Documentation/admin-guide/laptops/sonypi.rst
+ spectre_bhi= [X86] Control mitigation of Branch History Injection
+ (BHI) vulnerability. Syscalls are hardened against BHI
+ reglardless of this setting. This setting affects the
+ deployment of the HW BHI control and the SW BHB
+ clearing sequence.
+
+ on - unconditionally enable.
+ off - unconditionally disable.
+ auto - (default) enable only if hardware mitigation
+ control(BHI_DIS_S) is available.
+
spectre_v2= [X86] Control mitigation of Spectre variant 2
(indirect branch speculation) vulnerability.
The default operation protects the kernel from
@@ -2629,6 +2629,31 @@ config GDS_FORCE_MITIGATION
If in doubt, say N.
+choice
+ prompt "Clear branch history"
+ depends on CPU_SUP_INTEL
+ default SPECTRE_BHI_AUTO
+ help
+ Enable BHI mitigations. BHI attacks are a form of Spectre V2 attacks
+ where the branch history buffer is poisoned to speculatively steer
+ indirect branches.
+ See <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
+config SPECTRE_BHI_ON
+ bool "on"
+ help
+ Equivalent to setting spectre_bhi=on command line parameter.
+config SPECTRE_BHI_OFF
+ bool "off"
+ help
+ Equivalent to setting spectre_bhi=off command line parameter.
+config SPECTRE_BHI_AUTO
+ bool "auto"
+ help
+ Equivalent to setting spectre_bhi=auto command line parameter.
+
+endchoice
+
endif
config ARCH_HAS_ADD_PAGES
@@ -457,6 +457,7 @@
* Reuse free bits when adding new feature flags!
*/
#define X86_FEATURE_BHI_CTRL (21*32+ 2) /* "" BHI_DIS_S HW control available */
+#define X86_FEATURE_CLEAR_BHB_HW (21*32+ 3) /* "" BHI_DIS_S HW control enabled */
/*
* BUG word(s)
@@ -1541,6 +1541,74 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
dump_stack();
}
+/*
+ * Set BHI_DIS_S to prevent indirect branches in kernel to be influenced by
+ * branch history in userspace. Not needed if BHI_NO is set.
+ */
+static bool __init spec_ctrl_bhi_dis(void)
+{
+ if (!boot_cpu_has(X86_FEATURE_BHI_CTRL))
+ return false;
+
+ x86_spec_ctrl_base |= SPEC_CTRL_BHI_DIS_S;
+ update_spec_ctrl(x86_spec_ctrl_base);
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_HW);
+
+ return true;
+}
+
+enum bhi_mitigations {
+ BHI_MITIGATION_OFF,
+ BHI_MITIGATION_ON,
+ BHI_MITIGATION_AUTO,
+};
+
+static enum bhi_mitigations bhi_mitigation __ro_after_init =
+ IS_ENABLED(CONFIG_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON :
+ IS_ENABLED(CONFIG_SPECTRE_BHI_OFF) ? BHI_MITIGATION_OFF :
+ BHI_MITIGATION_AUTO;
+
+static int __init spectre_bhi_parse_cmdline(char *str)
+{
+ if (!str)
+ return -EINVAL;
+
+ if (!strcmp(str, "off"))
+ bhi_mitigation = BHI_MITIGATION_OFF;
+ else if (!strcmp(str, "on"))
+ bhi_mitigation = BHI_MITIGATION_ON;
+ else if (!strcmp(str, "auto"))
+ bhi_mitigation = BHI_MITIGATION_AUTO;
+ else
+ pr_err("Ignoring unknown spectre_bhi option (%s)", str);
+
+ return 0;
+}
+early_param("spectre_bhi", spectre_bhi_parse_cmdline);
+
+static void __init bhi_select_mitigation(void)
+{
+ if (bhi_mitigation == BHI_MITIGATION_OFF)
+ return;
+
+ /* Retpoline mitigates against BHI unless the CPU has RRSBA behavior */
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
+ !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
+ return;
+
+ if (spec_ctrl_bhi_dis())
+ return;
+
+ if (!IS_ENABLED(CONFIG_X86_64))
+ return;
+
+ if (bhi_mitigation == BHI_MITIGATION_AUTO)
+ return;
+
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
+ pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
+}
+
static void __init spectre_v2_select_mitigation(void)
{
enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -1652,6 +1720,9 @@ static void __init spectre_v2_select_mitigation(void)
mode == SPECTRE_V2_RETPOLINE)
spec_ctrl_disable_kernel_rrsba();
+ if (boot_cpu_has(X86_BUG_BHI))
+ bhi_select_mitigation();
+
spectre_v2_enabled = mode;
pr_info("%s\n", spectre_v2_strings[mode]);
@@ -2664,6 +2735,21 @@ static char *pbrsb_eibrs_state(void)
}
}
+static const char * const spectre_bhi_state(void)
+{
+ if (!boot_cpu_has_bug(X86_BUG_BHI))
+ return "; BHI: Not affected";
+ else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_HW))
+ return "; BHI: BHI_DIS_S";
+ else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))
+ return "; BHI: SW loop";
+ else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
+ !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
+ return "; BHI: Retpoline";
+
+ return "; BHI: Vulnerable (Syscall hardening enabled)";
+}
+
static ssize_t spectre_v2_show_state(char *buf)
{
if (spectre_v2_enabled == SPECTRE_V2_LFENCE)
@@ -2676,13 +2762,15 @@ static ssize_t spectre_v2_show_state(char *buf)
spectre_v2_enabled == SPECTRE_V2_EIBRS_LFENCE)
return sysfs_emit(buf, "Vulnerable: eIBRS+LFENCE with unprivileged eBPF and SMT\n");
- return sysfs_emit(buf, "%s%s%s%s%s%s%s\n",
+ return sysfs_emit(buf, "%s%s%s%s%s%s%s%s\n",
spectre_v2_strings[spectre_v2_enabled],
ibpb_state(),
boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? "; IBRS_FW" : "",
stibp_state(),
boot_cpu_has(X86_FEATURE_RSB_CTXSW) ? "; RSB filling" : "",
pbrsb_eibrs_state(),
+ spectre_bhi_state(),
+ /* this should always be at the end */
spectre_v2_module_string());
}